March 18, 2015 – NASA’s robotic MESSENGER spacecraft made history on March 17, 2011 when it became the first spacecraft to orbit the innermost planet. Over the last four years, its instruments, including a spectrometer built by the Laboratory for Atmospheric and Space Physics in Boulder, have fully mapped Mercury’s surface and yielded discoveries that have changed views on how the inner planets formed and evolved.

“MESSENGER truly is an extraordinary mission,” said Peter Bedini, a senior management advisor and former project manager for the mission. “When it began its primary mission four years ago, the spacecraft had already traveled almost five billion miles and completed six planetary encounters. During its pre-orbital Mercury flybys alone, MESSENGER collected as much data as Mariner 10 — the only other spacecraft ever to visit the innermost planet — and began addressing questions about Mercury that had remained unanswered for more than 30 years.”

“All objectives of the primary mission were easily met in the first year in orbit, and those findings stimulated questions that guided two successive extended missions,” added Bedini.

The latest findings were discussed in 15 papers presented this week and highlighted in a press conference at the 46th Lunar and Planetary Science Conference in The Woodlands, Texas.

The mission has also contributed to NASA’s technology base, said David Grant, who served as the mission’s project manager for five years, overseeing the development, integration, testing, and launch of the spacecraft and subsequent mission operations, including the Earth flyby and two Venus flybys.

Grant offers two examples. MESSENGER is the first mission to utilize “solar sailing,” that is, to correct the trajectory of the spacecraft with the Sun’s radiation, saving propellant and extending mission operations. The project is also the first to use SciBox, an automated science planning and commanding tool, for all data acquisition. Given spacecraft operational constraints and instrument operational constraints and objectives, the tool provides the science planner with an optimized set of opportunities to take observations and produces an integrated command sequence for the payload.

“But there was one other first among equals: system engineering,” Grant said. “I believe that the integration of the entire system presented the greatest challenge and led to our greatest success. Andy Santo, James Leary, Eric Finnegan, and Dan O’Shaughnessy served sequentially as MESSENGER’s Mission Systems Engineers over a period spanning more than 15 years. Their performance was exceptional and really speaks for itself.”

“The success of MESSENGER is a direct result of the talent and dedication of the team that designed and built it more than a decade ago, and of those who have operated it and directed the science data collection since launch in 2004,” said Helene Winters, MESSENGER’s current project manager. “The results of this Discovery-class mission have rewritten the book on Mercury and filled an important gap in our understanding of our Solar System.”

Now, after MESSENGER has logged more than 10 years in flight, the spacecraft and science instruments remain remarkably healthy, but the propulsion system is running on fumes. The force of solar gravity continues to perturb the spacecraft orbit in a manner that drives the probe downward toward the planet’s surface with each closest approach, and the tanks of propellant — needed to boost the spacecraft to higher altitudes — are running dry.

Engineers have devised a series of orbit-correction maneuvers (OCMs) to be performed over the next five weeks, which are designed to delay the inevitable impact a bit longer. The team has initiated this “hover” observation campaign to keep MESSENGER in orbit up to four weeks longer and to gather scientific data from the planet at ultra-low altitudes until the last possible moment. The first of these maneuvers was performed today.

A highly accurate OCM executed on January 21 targeting a 15-kilometer periapsis altitude — the lowest to date — set the stage for the hover campaign, in a short extension of the Second Extended Mission termed XM2-Prime (XM2′). The top science goals for XM2′ will be carried out with the Magnetometer (MAG) and the Neutron Spectrometer (NS), and each instrument will target different objectives in different regions, explained MESSENGER Deputy Project Scientist Haje Korth, of The Johns Hopkins University Applied Physics Laboratory (APL), in Laurel, Maryland.

Staying Aloft

The ever-present tug of the Sun’s gravity continues to perturb the spacecraft’s orbit and drive closest approach downward toward the planet surface. For the last few weeks MESSENGER’s altitude at closest approach has remained between 13 and 17 kilometers. To extend this hover campaign as long as possible, MESSENGER’s mission design team optimized the trajectory design and the placement of each orbit-correction maneuver.

“We decided on a strategy that includes five maneuvers in as many weeks to keep the spacecraft within a tight altitude range of 5 to 39 kilometers above the surface of Mercury at closest approach,” said APL’s Jim McAdams, MESSENGER’s Mission Design Lead Engineer.

Four of these five maneuvers occur in situations different from the dawn-dusk orbit orientation used for all earlier orbit-correction maneuvers in the mission, McAdams said. “During the interplanetary cruise phase, we designed similar course-correction maneuvers consisting of two or three separate, closely spaced maneuvers accomplished with different thruster sets. For XM2′, we simplified the design and implementation of the final maneuvers, so that each will be executed at a single spacecraft orientation using one thruster set to maximize the orbit altitude change per unit mass of propellant consumed.”

The maneuvers are not without risk, McAdams explained. “Increased uncertainty associated with effects on the spacecraft orbit of Mercury’s gravity field at lower-than-ever altitudes, challenges in accurately predicting the spacecraft orbit when the Sun is near the spacecraft-to-Earth communications direction, and implementation of frequent OCMs make for a challenging final few weeks of flight operations,” he said. “Depending on how each maneuver goes and on how Mercury’s gravity field affects the minimum orbital altitude, we may need to plan and implement a contingency maneuver. Inserting a contingency maneuver will increase the likelihood of the hydrazine propellant running out earlier than planned.”

So Far, So Good

This first maneuver went as planned. At the time of this most recent operation, MESSENGER was in an orbit with a closest approach of 11.6 kilometers (7.2 miles) above the surface of Mercury. With a velocity change of 3.07 meters per second (6.87 miles per hour), the spacecraft’s four largest monopropellant thrusters (with a small contribution from four of the 12 smallest monopropellant thrusters) nudged the spacecraft to an orbit with a closest-approach altitude of 34.5 kilometers (21.4 miles).

This maneuver also increased the spacecraft’s speed relative to Mercury at the maximum distance from Mercury, adding about 1.1 minutes to the spacecraft’s eight-hour, 16.5-minute orbit period. OCM-13 used propellant from the small auxiliary fuel tank.

MESSENGER was 185.6 million kilometers (115.4 million miles) from Earth when the 32-second maneuver began at 9:00 a.m. MDT. Mission controllers at APL verified the start of the maneuver 10.3 minutes later, after the first signals indicating spacecraft thruster activity reached NASA’s Deep Space Network tracking station in Goldstone, California.

The next maneuver, on April 2, will again raise the spacecraft’s minimum altitude, allowing scientists to continue to collect images and data from MESSENGER’s instruments.

MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) is a NASA-sponsored scientific investigation of the planet Mercury and the first space mission designed to orbit the planet closest to the Sun.

The MESSENGER spacecraft was launched a United Launch Alliance Delta II on August 3, 2004 and was inserted into orbit about Mercury on March 18, 2011 (UTC), to begin its primary mission — a yearlong study of its target planet. MESSENGER’s first extended mission began on March 18, 2012, and ended one year later. MESSENGER is now in a second extended mission, which is scheduled to conclude this spring.

Sean C. Solomon, the Director of Columbia University’s Lamont-Doherty Earth Observatory, leads the mission as Principal Investigator. The Johns Hopkins University Applied Physics Laboratory built and operates the MESSENGER spacecraft and manages this Discovery-class mission for NASA.

Simone Marchi of Boulder’s Southwest Research Institute has been an external collaborator of the NASA MESSENGER Geology Discipline Group. LASP contributed the Atmospheric and Surface Composition Spectrometer.